The invention relates generally to permanent magnets and more particularly to permanent magnets including rare earth elements, iron and boron as primary ingredients and improved methods of making those magnets.
Permanent magnets are important electronic materials and are used in a wide variety of fields ranging from household electrical appliances to peripheral console units of large computers. Higher performance standards have recently been required in permanent magnets. The demand for such magnets has also grown in proportion to the demand for small, high efficiency electrical appliances.
Typical known and commonly used permanent magnets include alnico magnets, hard ferrite and rare earth element--transition metal magnets. Rare earth element--transition magnets such as R-Co and R--Fe--B magnets provide particularly good magnetic performance.
Several methods have been developed for manufacturing rare earth iron based permanent magnets. These methods include:
1. A sintering method based on powder metallurgy techniques; PA1 2. A resin bonding technique using rapidly quenched ribbon fragments having thicknesses of about 30 .mu.m The ribbon fragments are prepared using a melt spinning apparatus of the type used for producing amorphous alloys; and PA1 3. A two-step hot pressing technique in which mechanical alignment treatment is performed on rapidly quenched ribbon fragments prepared using a melt spinning apparatus.
The sintering method is described in Japanese Patent Laid-Open Application No. 46008/1984 and in an article by M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matushita that appeared in Journal of Applied Physics, Vol. 55(6), p. 2083 (Mar. 15, 1984). As described therein, an alloy ingot is made by melting and casting. The ingot is pulverized to a fine magnetic powder having a particle diameter of about 3 .mu.m. The magnetic powder is kneaded with a binder such as a wax which functions as a molding additive. The kneaded magnetic powder is press molded in a magnetic field in order to obtain a molded body. The molded body, called a "green body", is sintered in an argon atmosphere for one hour at a temperature between about 1000.degree. and 1100.degree. C. and the sintered body is quenched to room temperature. Then the sintered body is heat treated at about 600.degree. C. in order to increase further the intrinsic coercivity of the body.
The sintering method requires pulverization of the alloy ingot to a fine powder. However, the R--Fe--B series alloy wherein R is a rare earth element is extremely reactive in the presence of oxygen. Thus, the alloy powder is easily oxidized when the oxygen concentration of the sintered body is increased to an undesirable level. When the kneaded magnetic powder is molded, wax or additives such as, for example, zinc stearate are required. While efforts have been made to eliminate the wax or additive prior to the sintering process, some of the wax or additive inevitably remains in the magnet in the form of carbon, which causes deterioration of the magnetic performance of the R--Fe--B alloy magnet.
Following the addition of the wax or molding additive and the press molding, the green or molded body is fragile and difficult to handle. Accordingly, it is difficult to place the green body into a sintering furnace without breakage and this is a major disadvantage of the sintering method. As a result of these disadvantages, expensive equipment is necessary in order to manufacture R--Fe--B series magnets according to the sintering method. Additionally, productivity is low and manufacturing costs are high. Therefore, the potential benefits of using inexpensive raw materials of the type required are not realized.
The resin bonding technique using rapidly quenched ribbon fragments is described in Japanese Patent Laid-Open Application No. 211549/1984 and in an article by R. W. Lee that appeared in Applied Physics Letters, Vol. 46(8), p. 790 (Apr. 15, 1985). Ribbon fragments of R--Fe--B alloy are prepared using a melt spinning apparatus spinning at an optimum substrate velocity. The fragments are ribbon shaped, have a thickness of up to 30 .mu.m and are aggregations of grains having a diameter of less than about 1000 .ANG.. The fragments are fragile and magnetically isotropic, because the grains are distributed isotopically. The fragments are crushed to yield particles of a suitable size to form the magnet. The particles are then kneaded with resin and press molded at a pressure of about 7 ton/cm.sup.2. Reasonably high densities (-85 vol %) have achieved at the pressure in the resulting magnet.
The vacuum melt spinning apparatus used to prepare the ribbon fragments is expensive and relatively inefficient. The crystals of the resulting magnet are isotropic resulting in low energy product and a non-square hysteresis loop. Accordingly, the magnet has undesirable temperature coefficients and is impractical.
Alternatively, the rapidly quenched ribbon or ribbon fragments are placed into a graphite or other suitable high temperature die which has been preheated to about 700.degree. C. in a vacuum or inert gas atmosphere. When the temperature of the ribbon or ribbon fragments has risen to 700.degree. C., the ribbons or ribbon fragments are subjected to uniaxitial pressure. It is to be understood that the temperature is not strictly limited to 700.degree. C., and it has been determined that temperatures in the range of 725.degree. k .+-.25.degree. C. and pressures of approximately 1.4 ton/cm.sup.2 are suitable for obtaining magnets with sufficient plasticity. Once the ribbons or ribbon fragments have been subjected to uniaxitial pressure, the grains of the magnet are slightly aligned in the pressing direction, but are generally isotropic.
A second hot pressing process is performed using a die with a larger cross-section. Generally, a pressing temperature of 700.degree. C. and a pressure of 0.7 ton/cm.sup.2 are used for a period of several seconds. The thickness of the materials is reduced by half of the initial thickness and magnetic alignment is introduced parallel to the press direction. Accordingly, the alloy becomes anisotropic. By using this two-step hot pressing technique, high density anisotropic R--Fe--B series magnets are provided.
In this two-step hot pressing technique, which is described in Japanese Laid-Open Application No. 100402/1985, it is preferable to have ribbons or ribbon fragments with grain particle diameters that are slightly smaller than the grain diameter at which maximum intrinsic coercivity would be exhibited. If the grain diameter prior to the procedure is slightly smaller than the optimum diameter, the optimum diameter will be realized when the procedure is completed because the grains are enlarged during the hot pressing procedure.
The two-step hot pressing technique requires the use of the same expensive and relatively inefficient vacuum melt spinning apparatus used to prepare the ribbon fragments for the resin bonding technique. Additionally, the two-step hot working of the ribbon fragments is inefficient even though the procedure itself is unique.
Finally, a liquid dynamic compaction process (LCD process) of the type described in T. S. Chin et al., Journal of Applied Physics, Vol. 59(4), p. 1297 (Feb. 15, 1986) can be used to produce an alloy having a coercive force in a bulk state. However, this process also requires expensive equipment and exhibits poor productivity.
Accordingly, it is desirable to provide a method of manufacturing improved rare earth-iron series permanent magnets that minimizes the disadvantages of the prior art methods.